400 years into the future, disease has wiped out the majority of the world's population, except one walled city, Bregna, ruled by a congress of scientists. When Æon Flux, the top operative in the underground 'Monican' rebellion, is sent on a mission to kill a government leader, she uncovers a world of secrets.

When her scientist ex-boyfriend discovers a portal to travel through time -- and brings back a 19th-century nobleman named Leopold to prove it -- a skeptical Kate reluctantly takes responsibility for showing Leopold the 21st century. The more time Kate spends with Leopold, the harder she falls for him. But if he doesn't return to his own time, his absence will forever alter history.